Arsenic (As) contamination of soils and water occurs worldwide-including in the U.S.-originating from both environmental and anthropogenic sources. Naturally-occurring alluvial As contaminates groundwater in South and Southeast Asia, and has caused wide-scale poisoning of a susceptible and malnourished populace. Chronic As exposure affects over 25 million people in Bangladesh alone. Industrial sources of arsenic-e.g., arsenic- containing pesticides in agriculture-also contribute to global exposure. In the United States (US), As tops the hazardous substance priority list of the Agency for Toxic Substances and Disease Registries, and 13 million Americans are exposed to levels exceeding the US Water Quality Standards. Chronic exposure leads to arsenicosis, which includes melanosis, keratosis, and cardiovascular effects, as well as lung, bladder, kidney, and skin cancer. Chronic As exposure can also affect childhood intellectual function and development. There is no effective therapy for arsenicosis, emphasizing the importance of prevention and/or novel therapeutic approaches. Prevention requires effective risk-assessment, which also lays the groundwork for intervention design and treatment. The central hypothesis of this proposal is that As ingestion alters the structure and function of the human gut microbiota and that differences in the human gut microbiota contribute to the observed variation in the presence/absence of arsenicosis, given the same level and route of As environmental exposure through groundwater and rice. To test this hypothesis, the principal investigators will pursue the following specific aims: Specific Aim : To determine the effects of exogenous As on the gut microbiota and on the resulting transformation of As using anaerobic bioreactors in the absence/presence of arsenic-amended media; Specific Aim 2: To characterize the frequency and diversity of microbial genes related to As transformation in the gut microbiota of healthy humans with no As exposure, as well as of exposed individuals with/without arsenicosis; and Specific Aim 3: To characterize variations in taxonomic composition and functional potential of the gut microbiota of arsenic-exposed individuals with and without arsenicosis. To accomplish these specific aims they will make use of fecal specimens from non-exposed, healthy human individuals from the US, as well as fecal samples from subjects in Bangladesh with (1) chronic As exposure and arsenicosis, (2) chronic exposure and no arsenicosis, or (3) no exposure and no arsenicosis (control group). This multidisciplinary project lays the groundwork for microbiome-related risk assessment and further studies which will inform novel interventions for arsenicosis, a disease affecting millions worldwide.